<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<script>
    document.write(isNaN(NaN)+"<br>");
    document.write(isNaN("blue")+"<br>");
    document.write(isNaN("123")+"<br>");
    document.write(isNaN(null)+"<br>");
    document.write(isNaN(undefined));
</script>

</body>
</html>